Three major conventional metal pattern forming methods known in the art are “subtractive method”, “semi-additive method”, and “fully-additive method”.
The subtractive method includes steps of forming a photosensitive layer sensitive to activated light irradiation over a metal layer formed on a substrate, exposing the photosensitive layer imagewisely, forming a resist image by developing, forming a metal pattern by etching the metal, and finally removing the resist. The substrate interface of the metal substrate used by this method has often been roughened to provide an anchoring effect for improvement in the adhesiveness between the substrate and the metal layer. As a result, the substrate interface of the metal pattern formed became rough, often causing a problem of deterioration in high frequency characteristics when the product is used for electric wiring. In addition, it also carried the problem of requiring a complicated step of treating the substrate with a strong acid such as chromic acid or the like, as surface roughening of the substrate is required before forming the metal substrate.
To overcome these problems, for example, a method of providing a certain degree of adhesiveness by subjecting the substrate surface to a surface grafting treatment and thus simplifying the processing steps of substrate and a method of modifying the substrate surface by grafting a radical polymerizable compound thereon, thus minimizing the roughness of the substrate and accordingly simplifying the substrate processing step were proposed in Japanese Patent Application Laid-Open (JP-A) Nos. 58-196238, and Advanced Materials 20, pp. 1481-1494, 2000. However, an expensive apparatus such as a γ-ray generating apparatus and an electron beam generating apparatus is necessary. Further, it is predictable that a polymerization initiating group which causes graft polymerization is not introduced into the substrate, resulting in small amount of a graft polymer to be formed.
Accordingly, even if a metal substrate prepared by this method is patterned by the subtractive method, the product still contained a problem inherent to the subtractive method as follows. For forming a metal pattern having a thinner line width by the subtractive method, so-called over-etching method, whereby the line width of a resist pattern becomes thinner after etching, is effective. However, when a fine metal pattern is directly formed by the over-etching method, it was difficult to form a metal pattern having a line width of 30 μm or less from the practical viewpoint of forming a favorable fine metal pattern, due to the defects of the wire formed such as bleeding, thinning, disconnection, and the like. In addition, it also carried the problems in cost and environmental friendliness, as it demanded removal of the metal film present in the area other than the patterned area, and it also required a significant cost for processing the metal wastewater from the etching treatment.
To overcome these problems, a new metal pattern forming method, called semi-additive method, was proposed. The semi-additive method is a method including following steps. That is, a thin metal underlayer of Cr or the like is formed on a substrate by plating or the like, and then a resist pattern is formed on the metal underlayer. A wiring pattern is produced by forming metal layer of Cu or the like on the region of the metal underlayer other than the resist patterned area by plating and then removing the resist pattern. A metal pattern is subsequently formed on the region other than the resist patterned area by etching the metal underlayer while masking the wiring pattern. The method allows easier formation of a thin line pattern having a line width of 30 μm or less, as it is an etching-less process; and it is also advantageous in cost, as the metal is deposited only on the area required. However, the method demands roughening of substrate surface for improvement in the adhesiveness between the substrate and the metal pattern, and as a result, the substrate interface of the metal pattern formed became rough, causing a problem of deterioration in the high frequency characteristics of product when it is used for electric wiring.
Alternatively, a metal pattern forming method called fully-additive method was also proposed. The fully-additive method is a method of forming a resist pattern on a substrate by depositing a metal on the region other than the resist pattern by plating and then removing the resist pattern. The fully-additive method allows easier formation of a thin line pattern having a line width of 30 μm or less as it is an etching-less process; and it also made the substrate interface rough in a similar manner to the semi-additive method, causing a problem of deterioration in the high frequency characteristics of product when it is used for electric wiring.
As described above, a metal pattern forming method allowing formation of a thin line pattern, having smaller substrate interface roughness, and producing a smaller amount of etching wastewater is yet to be proposed, and there existed a need for a new metal pattern forming method
The metal pattern described above is useful as a wiring (conductive film) for a printed wiring board for a semiconductor device. In recent year, for electronics, there has been a high demand for processing mass storage data at high processing rate. Internal clock frequencies or external clock frequencies in a semiconductor device for processing images or controlling processing increase year by year, and the number of connecting pins also increases. In order to carry out high-speed conduction, it is important to reduce delay and damping of signals. In order to reduce propagation delay of signals, it is effective to decrease dielectric constant. In order to reduce dielectric loss, it is effective to decrease dielectric constant and dielectric tangent, respectively. However, since dielectric constant in dielectric loss is the root of dielectric constant, in actuality, dielectric tangent is largely concerned with this. For this reason, from a viewpoint of material characteristics, it is advantageous to adopt insulating materials having low dielectric tangent characteristics for carrying out high speed data processing.
Further, surface smoothing of conductive materials substantially contributes to forming materials with high density. In a conventional built-up printed wiring board, roughening treatment has been employed for obtaining stripping strength. However, status quo, roughness of several microns interferes finer wiring.
Accordingly, from a viewpoint of forming a printed wiring board useful for a semiconductor device, means for forming very fine metal patterns with high adhesiveness on a smooth insulating substrate.